This invention is generally directed to a novel wheel speed sensor for a vehicle, such as a truck or a trailer.
Anti-lock brake systems (ABS) for air braked truck trailers are in common use and generally include an Electronic Control Module (ECM), Pneumatic Control Module (PCM) and wheel speed sensors. The various components may be known by different names, for example the ECM is sometimes called the Electronic Control Unit (ECU). The PCM may be known as a modulator. Some ABS implementations integrate the PCM into another component known as the relay valve.
The wheel speed sensors provide wheel speed information to the ECM and the ECM signals the PCM to modify air pressure level at the brake chambers. This whole process controls the braking level so that the wheels continue to rotate, or at least rotate most of the time, even during heavy braking. The overall process is described in detail in numerous patents and in co-pending U.S. patent application Ser. No. 09/306,921 which is commonly owned by the assignee herein.
As noted, the wheel speed sensors determine the speed of the wheels and send this information, in an electrical format, to the ECM. Typically, current production ABS systems for heavy trucks and trailers use what are known as variable reluctance (VR) sensors. They are based on well-established technology and are available from numerous suppliers.
The current industry standard sensing mechanism uses a VR sensor together with a toothed ring which is fitted to the back of a wheel hub in the wheel mounting assembly. The VR sensor consists of a magnet, a coil of wire and a metal (soft iron) pole piece, all of which are enclosed in a cylindrical casing of about 2xc2xd inches long and ⅝ inches in diameter. The toothed ring generally has one hundred teeth. As the wheel rotates, magnetic flux through the coil of wire in the VR sensor is present to a greater or lesser degree depending on whether or not a tooth from the toothed ring is directly over the pole piece in the face of the VR sensor. This changing flux induces an alternating voltage in the coil of wire. Its frequency is proportional to the speed of the wheel and the proportionality relationship is about 14 Hertz (cycles per second) per mph. The frequency signal is used by the ECM to determine the speed of the wheel. The amplitude of the signal also increases with wheel speed but the amplitude information is not used directly by the ECM.
The VR sensor is located in a bore in a metal block which is welded to the axle behind the wheel hub. A sleeve within the bore retains the VR sensor so that the face of the VR sensor remains close to the toothed ring. The VR sensor is installed by pushing the VR sensor through the bore from behind until the face contacts the toothed ring. In service, axial movements of the wheel hub pushes the VR sensor back several thousandths of an inch so that most of the time the face of the VR sensor does not actually touch the toothed ring.
The waveform generated by the VR sensor is approximately sinusoidal. As noted above, its frequency is directly proportional to wheel speed and its amplitude also increases as speed increases. The amplitude is also highly dependent on the gap between the face of the VR sensor and the toothed ring.
The VR sensors used in the prior art do not measure xe2x80x9czeroxe2x80x9d speed. The VR sensors generate voltage in response to changing magnetic flux but the VR sensors do not respond to absolute flux levels. As such the VR sensors cannot determine whether the pole of the VR sensor is opposite a tooth or opposite a gap if the toothed ring is stationary. Very slow rates of rotation, for example below one mph depending on sensor gap, do not generate enough alternating voltage for the ECM to determine that rotation is actually occurring.
Passenger car ABS sensing technology sometimes uses xe2x80x9cactivexe2x80x9d sensors. They have a semiconductor type element to replace the coil of wire in conventional VR sensors. This semiconductor type element measures the actual level of magnetic flux, not a rate of change. These active sensors can measure close to xe2x80x9czeroxe2x80x9d speed as the active sensors respond to each change from a tooth to a gap, or vice versa, regardless of speed. For active sensors, the toothed ring is sometimes replaced by a circular multi-pole magnet which may be considered to be an exciting element or ring.
In addition to the semiconductor type sensing element, the active sensors also require integrated electronic circuitry to amplify the signal to reasonable levels for transmission to the ECM. Hence, active sensors use several xe2x80x9cactivexe2x80x9d electronic components. Either a two or a three-wire connection to the ECM is required. Approximate square wave signals with levels of zero and five volts for a three-lead design and with a current variation of several milliamps for a two-lead design are typical.
From a functional perspective, xe2x80x9czeroxe2x80x9d speed sensing, of itself, does not provide significant advantage for a simple trailer ABS product. However in certain circumstances, this active technology provides implementation advantages. Also, as will be discussed here, this technology allows easier implementation of certain features which can be added to a basic trailer ABS product.
With current practice in the heavy truck industry, the sensor 10 is mounted axially and the face of the sensor 10 is perpendicular to the axle 11, see FIG. 7. This arrangement results in a number of technical imperfections, particularly in the context of VR sensing technology. The state of bearing adjustment determines how precisely the wheel hub 12 is constrained in an axial direction. Thus if the bearing adjustment is allowed to become sloppy, the wheel hub 12 can move axially in service. In fact, certain bearing adjustment procedures require some degree of allowed axial movement. The axial movement is sometimes called xe2x80x9cendplayxe2x80x9d in the heavy truck industry. For example, a requirement for 0.008 inches of endplay is not uncommon. Axial slack results in an increased gap between the face of the sensor 10 and the toothed ring 13. The first effect is a lower amplitude signal. In addition, rapid in-service axial movements of the wheel hub 12 can occur due to tire impact with potholes and other imperfections. Even cornering forces can cause significant axial movement. These effects result in major imperfections to the ideal sinusoidal voltage waveform from the sensor 10. Both amplitude modulation and lower frequency added components result from the varying gap.
Depending on the severity, these effects combine to the extent that the ECM cannot operate optimally in all circumstances and minor imperfections in control performance result. For even greater degradation, the ECM determines that the signal is unusable and ABS system shutdown and warning lamp illumination results.
Another issue with the prior art sensing systems is that the signal is sensitive to any run out in the toothed ring at the back of the wheel hub. If the plane of the face of the teeth is not perpendicular to the axis of rotation, the gap to the face of the sensor varies as the wheel rotates. This variation causes effects (amplitude modulation and lower added frequency components) similar to the loose bearing adjustment noted above. In this case, for a one kHz waveform (about seventy mph), the unwanted components are at a frequency of ten Hz.
The geometrical arrangement normally used for passenger car ABS systems is in contrast to that just described. The face of the sensor is normally parallel to the axle shaft. The bearings maintain a constant gap between the sensor and the toothed ring so the above mentioned defects are practically non-existent. The gap does not vary, except perhaps by a minimal amount, as axial motion of the wheel hub occurs.
In general, it is required to have relative rotation between an element similar to the toothed ring and an element similar to the sensor itself. It is also clear that connecting wires are needed to carry the signal back to the ECM. In the context of a trailer axle, the relative motion is between the wheel hub and the axle. The need for connecting wires implies, in practice, that the sensing element is stationary and that the toothed ring rotates. It is clear that the conventional approach with a toothed ring at the back of the wheel hub and a stationary sensor attached to the axle does meet these basic requirements. However, this arrangement has the drawbacks mentioned herein. The present invention provides a novel alternative this system.
In a prior system, a toothed ring 14 is attached to the inside of a special hub cap 15 and rotates with the hub cap 15, see FIG. 8. The sensing element 16 is attached to the end of the stationary hollow axle 17 and wires exit through the axle 17.
In principal, very good technical performance is achievable from such sensing elements 16. Similar to passenger car ABS technology, the sensing face is parallel to the axis of rotation. Consequently, a constant gap is maintained by the bearings 18. Axial movement of the wheel hub 19 does not have a significant effect. No gap adjustment is required. The gap is set by design, and gap variation is directly controlled by the bearings 18. However, the gap is dependent on the concentricity of the mounting of the toothed ring 14 within the hub cap 15. The use of multiple coils of wire and/or pole pieces in the sensing element 16 may be used to provide partial electrical compensation for run out or other imperfections in the construction or mounting of the toothed ring 14.
However, to remove the wheel hub 19, the mechanic has to remove and reinstall the sensing element 16, including wiring connections. Personnel who are involved in wheel end maintenance now have to pay attention to another set of requirements including the handling of delicate sensing elements and wiring connections. As already noted, the eccentricity of the toothed ring 14 may be difficult to control. It is believed that the toothed ring in a prior art sensor was fabricated from a stamped band which was subsequently formed into a circle. Partial electrical compensation is possible in the sensing element at added cost.
The present invention provides a novel configuration for a wheel speed sensor for a vehicle, such as a trailer which overcomes the problems presented in the prior art. This and other features and advantages will become apparent upon a reading of the attached specification in combination with a study of the drawings.
A general object of the present invention is to provide a novel configuration for a wheel speed sensor for a vehicle, such as a truck or a trailer.
An object of the present invention is to provide a wheel speed sensor which allows the wheel mounting apparatus to be removed from the axle without removing the wheel speed sensor.
Another object of the present invention is to provide a wheel speed sensor which has a sensing face parallel to the axis of rotation of the axle and senses an exciting, element which is radially spaced therefrom, such that the gap between the wheel speed sensor and the exciting element is maintained by the wheel bearings.
A further object of the present invention to provide a wheel speed sensor which is not significantly effected by movement of the wheel hub.
An even further object of the present invention is to provide a wheel speed sensor which does not require gap adjustment.
Briefly, and in accordance with the foregoing, the present invention discloses a system for a vehicle, such as a truck or a trailer. The system includes a hollow axle, a wheel hub which surrounds the axle, a plurality of bearings provided between the axle and the wheel hub, at least one nut mounted on an end of the axle, and a hub cap mounted to the wheel hub. The nut has a predetermined inner diameter and is used to properly position the bearings on the axle along with structure on the axle and the hub cap. An exciting element is mounted within the hub cap by a mounting wheel. A sensor member is mounted in the end of the axle and radially spaced from the exciting ring. The sensor member senses the exciting ring and sinks current from a current supplying controller on the vehicle via suitable wiring. The controller determines the speed of rotation of the wheel hub by determining the magnitude of the current sink. The sensor member has an outer diameter which is less than the inner diameter of the nut such that the wheel hub, the bearings, the nut, the hub cap, the exciting ring and the mounting wheel can be removed without having to remove the sensor member from the axle.